It has long been the practice to cover the firebox walls of facilities such as municipal incinerators with a firebrick or tile sheath in order to protect the structural elements thereof, the walls and the ceilings, from the erosive and corrosive effects of the fire. Many of these facilities now include energy recovery systems which operate to retrieve the heat generated during the combustion process. In many cases the energy recovery system comprises a boiler, including an array of metal tube walls through which water is circulated as a heat transfer medium. This array is often placed at the periphery of the firebox and is also susceptible to the detrimental effects of impinging flame. Therefore, it is desirable to protect this heat exchanger array with its own refractory covering which is suspended from the heat exchanger itself.
In contrast to the firebrick employed to protect the structural elements of the facility, the refractory material used to cover the heat exchanger is seldom simply firebrick held together with mortar but is often a more specialized type of brick or tile with unique characteristics. One of the characteristics required of the brick is high thermal conductivity to the underlying heat exchanger. Another, but related, special requirement is a means for attaching the brick to the heat exchanger. Attachment of the brick has been accomplished in a number of ways.
In U.S. Pat. No. 3,327,445, refractory bricks are suspended on the vertical wall of a furnace on metallic support shoes and hung from vertical "I" beams using "J" bolts anchored in the bricks. U.S. Pat. Nos. 3,328,014 and 3,380,409 disclose furnace wall construction in which tongue and groove mating elements are used to hang refractory brick from a vertical metal framework. In none of these references is the brick hung from a heat exchanger.
U.S. Pat. No. 1,987,738 discloses a steam locomotive firebox which contains an array of tubing through which water is circulated. The heat exchanger on the metallic arch of the firebox, which is inclined from the vertical, is shielded from direct flame with a layer of refractory blocks. The blocks are suspended from and anchored to the array by means of mating tongue and groove structures in the blocks and arch. Boiler tubes in a furnace are protected with refractory brick which is hung from the tubing framework with mated fittings according to U.S. Pat. Nos. 3,850,146 and 3,828,735.
U.S. 3,797,416 discloses protecting boiler tubes from the erosive effects of injected pulverized coal fuel by hanging protective structures from the tubing using mated ear and groove fittings which are wedged together to provide a tight fit to the tubing. Nickel alloy, not refractory brick, is disclosed as the protective structural material, flame not being the erosive agent.
U.S. Pat. No. 4,768,447 describes a firebrick adapted to be hung on a vertical wall of tubing in an incinerator by means of an inclined recess in the firebrick into which a mating projection from the tubing wall fits; the firebrick is held against the tubing by gravity.
Whereas there have been many proprietary methods for hanging protective refractory brick on an array of heat exchanger tubing, few of these methods have commercial significance. The most common method for hanging the firebrick is to run a bolt through the brick and anchor it to the tubing. This technique, while relatively inexpensive, subjects the brick to compressive stresses, which leads to the development of cracks and, ultimately, to failure of the brick. Furthermore, while many of the methods for affixing the bricks may be satisfactory when the array of heat exchanger tubing is vertical, they become unsatisfactory for use when the array is inclined from the vertical toward horizontal; the weight of the bricks then tends to pull them away from the tubing, lowering the thermal conductivity drastically and sharply affecting the efficiency of heat recovery.
For example, the firebrick of U.S. Pat. No. 4,768,447 is forced against the tubing array by gravity only to the extent the array is vertical. To the degree the array is inclined from the vertical, the weight of the brick tends to separate it from the tubing.